Certain polymers, such as polydiene elastomers, exhibit cold flow at standard conditions. In other words, the polymers flow under their own weight, which causes problems when attempting to transport or store the polymers. Therefore, it is desirable to prevent cold flow from occurring by improving cold flow resistance of the polymer.
One solution employed in the art includes coupling the polymers. For example, linear polydienes, such as those polydienes produced by anionic polymerization or coordination catalysis, have been coupled with compounds. Although coupling agents may serve to improve cold flow resistance, they do not always serve to provide desired tire properties.
Therefore, in the art of making tires, particularly tire treads, functionalized polymers are advantageously employed to improve properties such as hysteresis loss. These functionalized polymers are often prepared by terminating growing polymer chains with functionalizing agents that impart a functional group on the end of the polymer chain.
Unfortunately, the functionalizing agents employed to terminate the polymer and provide advantageous properties to the tire do not always serve to provide cold flow resistance to the polymer. Furthermore, coupling agents, which improve cold flow resistance, are often incompatible with functionalizing agents while reacting with the polymer chain. Therefore, it is not always possible to combine functionalizing agents and coupling agents to achieve both good cold flow resistance and desired tire properties.
Because there remains a need for functionalized polymers, and in fact there is a desire for functionalized polymers with a high degree of functionality, the use of coupling agents that compete with the functionalizing agent can be detrimental to this goal. A need therefore exists to improve cold flow resistance by means other than coupling reactions.